Sierpiński arrowhead curve

en.wikipedia.org/wiki/L-system#Example_5:_Sierpinski_triangle

#fractal #lsystem

Created by reinder on 2018/11/2
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// Global code will be evaluated once.
const turtle = new Turtle(-90,-75);

// l-system
function createLSystem(numIters, axiom) {
    let s = axiom;
    for (let i=0; i<numIters; i++) {
        s = processString(s);
    }
    return s;
}

function processString(oldStr) {
    let newstr = "";
    for (let i=0; i<oldStr.length; i++) {
        newstr += applyRules(oldStr[i]);
    }
    return newstr;
}

function applyRules(ch) {
    switch (ch) {
        case "A": return "B-A-B"; //   # Rule 1
        case "B": return "A+B+A"; //   # Rule 2
        default: return ch;
    }
}

const inst = createLSystem(8, "A"); // number of iterations and axiom
const distance = .7;
const angle = 60;

// The walk function will be called until it returns false.
function walk(i) {
    const cmd = inst[i];
    
    switch (cmd) {
        case "A":   turtle.forward(distance);
                    break;
        case "B":   turtle.forward(distance);
                    break;
        case "+":   turtle.right(angle);
                    break;
        case "-":   turtle.left(angle);
                    break;
        default:
    }
      
    return i < inst.length - 1;
}